The present disclosure relates generally to apparatuses and control methods for power factor correction, more particularly to apparatuses and control methods for improving the power factor when a power supply drives a light load.
The power factor of an AC electrical power system is defined as the ratio of the real power flowing to a load, to the apparent power into the power circuit, and is a dimensionless number between −1 and 1. If a power supply has a power factor less than 1, an electric power company must reserve power delivery capacity more than the output power rating of the power supply, to make the power supply operate properly in all allowed circumstances. In order to relieve the burden of reserving over-high power delivery capacity, developed and developing countries have enforced regulations requiring power supplies for lighting or with more than 100 Watt output rating to have a power factor more than 0.9.
Active power factor correction (PFC) refers to use of active devices including a control circuit and at least one power switch for achieving a good power factor. Normally, one power switch is well controlled to intentionally drain an input current, making its average substantially in proportion to an input voltage, such that a power factor of 1 could be approximately achieved. Active PFC is commonly embodied by switching-mode power supplies (SMPSs). A booster operating in a constant ON-time scheme, for example, could results in a very high power factor.
A SMPS normally has an anti-EMI (electromagnetic inference) circuit, which usually includes a low-pass filter comprising an inductor and a capacitor at least, and is positioned between a main converter and an outlet plug connected to AC main power lines. When driving a light load or no load, the SMPS drains very little current from the outlet plug, and the main converter might receive a filtered input voltage very different from the input voltage in the outlet plug. As a result, even if the main converter makes its average input current substantially in proportion to the filtered input voltage, the power factor of the SMPS is not optimized because the waveform of the average input current still differs to that of the filtered input voltage in shape.
Therefore, it is an issue to optimize the power factor of a SMPS when driving a light load.